The present invention relates to a homodyne optical receiver equipment, and in particular to an optical receiver equipment, which is stable against variations in the polarization of optical signals.
In a homodyne optical receiver equipment it is necessary to make an optical signal input to the optical receiver equipment and an optical local signal emitted by an optical local oscillator incorporated in the optical receiver equipment match with each other in phase and polarization. This is because mismatch in the phase and the polarization can cause worsening in the reception sensitivity of the optical receiver equipment. Heretofore, no techniques have been proposed for polarization matching, while techniques for phase matching have been proposed. For example, in Reference 1 "ECO 89, Technical Digest, ThA 21-6, pp. 413-416, September 1989", Reference 2 "Electronics Letters, Vol. 26, No. 10, pp. 648-649, May 1990, etc. excellent methods for the phase matching have been proposed and verified. However, there are no proposals for polarization matching and in experiments polarization matching was effected by using a manual polarization controller. Further, in this reference, a part of a signal obtained by homodyne detection is branched off and phase matching is realized by controlling a semiconductor laser acting as an optical local oscillator while negatively feeding back thereto a current proportional to the branched signal, which has passed through a loop filter. Furthermore, it is known that such phase matching can be realized by controlling an optical phase controller, in which light output by the semiconductor laser is input, while feeding it back thereto.
For making a homodyne optical receiver equipment usable in practice it is necessary to add a function of automatically effecting the polarization matching to a phase matching function.